Conditioning the atmosphere in subterranean excavations



Oct. 15, 1946. J, REES 2,409,388

CONDITIONING THE ATMOSPHERE IN SUBTERRANEAN EXCAVATIONS Filed Aug. 18, 1942 w I v v v v INVENTQQ John PrIQQ, @QS

, (Eng 4 I; GL1 ATTTKI.

Patented Oct. 15, 1946 UNITED CONDITIONING THE ATMOSPHERE IN SUBTERRANEAN EXCAVATIONS John Price Rees, Johannesburg, Transvaal,

I Union of South Africa Application August 18, 1942, Serial No. 455,268 In the Union of South Africa July 4, 1942 6 Claims. (Cl. 98--50) This invention relates to conditioning the atmosphere of subterranean excavations and particularly deep mines. Essential requisites of such an atmosphere are that it be maintained in respirable condition in the sense of preserving the proper pro-portion of its physiologically active constituents to its diluent and physiologically inert constituents, and ensuring the removal of deleterious substances such as phthisical dust and gaseous products of blasting; and further that its temperature, with regard to its cooling efiect upon human beings working in it, be kept from rising to a level at which persons cannot work with a reasonable degree of efficiency and may even be in danger of physical collapse.

The maintenance of the atmosphere in the respirable condition mentioned above can be effectively carried out by ventilating currents of sufiicient volume drawn. from and returned to the earths atmosphere. v

As regards the heat problem the ventilating air undergoes ariseof temperature due to acquiring heat from two principal sources, viz. (a) heat inflowing from the rock, which is at atemperature higher than that of the earths atmosphere from which the ventilating current is derived; and (b) the heat due to the autocompression of the ventilating air. In the process of autocompression, the air descending the downcast shaft moves towards the centre of the earth and is thereby made denser by the -correspond ing increase of the mass of air lying above it; the energy which thus appears as increased denseness of the air and heat being the potential energy that the air has parted with during its descent.

Rise of temperature of the mine atmosphere due to cause (a) can be and is being kept within control by such means as making the split ventilating currents ample enough to sweep away the heated air as it is formed; and to someextent by employing heat pumps to extractthe heat from small local areas of the mine where work is actually going on and conveying away the extracted heat in a manner that avoids re-heating of the downcast air.

The control of temperature rise due to autocompression is however a problem of a different kind. The heat developed inthis way cannot for instance be swept out by increasing the ventilating current since the heat is developed in the ventilating air itself and each unit mass of air that descends the shaft develops its own quota of heat; the only variable factor that influences the amount of heat being the vertical depth of downcast travel. It is moreover independent of the kind of gases of which the Ventilating current is composed.

Some limitation of the temperature rise at the lower termination of the downcast system has in practice been attained by pre-cooling the ventilating air at the surface where the extracted heat can readily be disposed of to the earths atmosphere; but it seems that the limit of-temperature reduction that can economically be attained in this way amounts to about 10 degrees Fahrenheit; sufiicient, when the temperature gradient is of the order of ldegree Fahrenheit per 200 feet of depth, to extend the feasible depth oimining by about 4,000 feet.

The present invention attacks the autocompression problem in a different manner, by taking account of the fact that whilst the quantity of heat developed by autocompression is virtually-beyond control, thetemperature rise clue to the heat of autoco'mpression can be lessened by increasing the specific heat of the ventilating current. For this purpose there is substituted for the normal air that is passed intothe mine heat added to such substitute atmosphere the temperature rise is less, in correspondence with the greater capacitay for heat of the substitute atmosphere. More specifically, since any respirableatmosphere must contain oxygen and carbon dioxide in invariable proportions within small limits diluted with a physiologically inert gas, the change made by the invention consists in substituting the nitrogen of the normal atmosphere partly or completely by a similarly physiologically inert gas, but one which has a higher specific heat than nitrogen. Hydrogen for instance which has a specific heat about fourteen times that of nitrogen, and which is physiologically inert, could be used under conditions that would preclude chemical combination of the hydrogen with the necessary quota of oxygen. But under present circumstances the most suitable gas is helium, which has been V proved to be physiologically suitable, and even beneficial as compared with nitrogen; is available in large quantities; is inert chemically in regard to oxygenand carbon dioxide; and is obtainable at a cost that renders its use an economic possibility; Its specific heat is favourable; the specific heat of helium airviz. air in which nitrogen has been wholly replaced by helium-being 0.57 as compared with 0.24 for normal nitrogen air, at the same pressure. The temperature of helium air would therefore rise about 2.2 degrees Fahrenheit per 1,000 foot depth; apart from the cooling effect of evaporation from Wet surfaces. Such evaporation would cool helium air rather more than it cools an equal volume of ordinary air and the rise in wet bulb temperature would be less than with ordinary air. For instance at 12,000 feet the wet bulb temperature would be about 60 F. the surface wet bulb temperature being 50 F. M

Experience gained in connection with the Witwatersrand mines suggests the addition of about F. for uncontrolled heating in the shaft 1. e., a little from the rockwalls and the rock being hoisted, from hoists, from water being pumped out etc., giving an estimated temperature of 70 F. wet bulb on the shaft station on the average for the year.

The above calculation indicates that in order to extend the depth of mining to about 12,000 feet vertically below the surface, it would not be necessary to replace all the nitrogen in the air by helium. In practice the amount of helium may be adjusted to give comfortable conditions for mining with minimum outlay on helium. For example, gas mixture consisting of approximately 40% helium by volume, 40% nitrogen, oxgen', together with a small proportion of carbon dioxide would have a specific heat of 0.32. The wet bulb temperature would be, therefore, about 80 F. at 12,000 feet on the shaft station.

'In view of the monetary value of the helium it is important that the mine ventilating system be constructed as a closed circuit that retains the helium with a minimum of loss whilst providing for there-conditioning of the gas mixture coming from the'upcast to render it suitable for return to the mine. In particular, it is necessary to discharge from the exhaust current the heat it has acquired from the mine workings: to make up oxygen consumed and helium lost; and to remove deleterious substances. Such a circuit is shown in the accompanying drawing which is a diagrammatic section through the mine. Therein ID indicates workings in which persons are engaged in mining. II is a shaft which gives access to said working and is also the downcast shaft of the ventilating system. I2 is the upcast shaft of the system; and I3 is a ventilating fan which may be positioned at the foot of the upcast shaft with the object of setting up a plenum in the latter and thereby preventing leakage of normal air into the ventilating atmosphere.

I4 indicates the surface of the mine, where there is erected an enclosing structure [5 forming a duct through which the gas discharge from the upcast shaft I2 i passed to the downcast shaft II. Said structure l5 encloses the headgear I6 and is provided with an airlock I! through which trafiic to and from shaft H can pass with minimum leakage of gas into or from the closed circuit.

Said structure l5 also includes means indicated generally by [8 by which rock dust, smoke and soluble gases given off in the mine are removed by water sprays l9, which may also cool the air to some extent. 20 indicates further means through which the gas current passes for the addition thereto of make up oxygen and helium, and for the removal of excess carbon dioxide. Carbon monoxide is a constitutent of the exhaust current from a mine in which nitroglycerin and similar explosives, are used; but as that gas is difficult to remove from a ventilating current, it

is preferred in the present case to ensure its absence by using an innocuous explosive such as liquid oxygen for the purpose of breaking rock.

Further plant that may advantageously be included in the duct I5 is that indicated by 2!, the function of which is to effect the known surface cooling of the ingoing ventilating current with the object of lowering the temperature of the atmosphere in the mine workings. Without the reduction of temperature by means of the present invention it is not-as mentioned above-feasible to effect much reduction of underground temperatures by surface cooling without the great expense involved in providing a cooling plant using brine as the cooling medium; but the cooling brought about by the invention, added to the surface cooling which can be cheaply effected when the cooling medium is kept above the freezing temperature of water, enables a substantial cooling effect to be produced underground at moderate expense.

In the operation of the ventilating system, using helium air in which all the nitrogen is replaced by helium, the helium air would reach the surface at a higher temperature than ordinary upcast air due to the reversal in the upcast shaft l2 of the autocompression occurring in shaft H. For example, for a certain mine depth at which ordinary upcast air would be expected to arrive with a temperature of 70 to 75 F., the helium air would arrive with a temperature of about F. This excess over about 70 is however easily dealt with at low cost by sprays and cooling towers and without refrigeration. If the cooler 2! is arranged to cool the current from about 70 saturated to about 50 saturated, the resulting gas temperature at 12,000 feet would as a consequence be reduced by about 30 F. in the case of helium air as compared with about 10 F. in the case of ordinary air.

The conversion of an ordinary mine atmosphere to the atmosphere of the invention offers no outstanding difiiculty since it involves no change in the respirability of the atmosphere. It wouldbe necessary to continue the normal procedure of discharging the upcast ventilating current to the open air while the helium is being introduced into the downcast current. Loss of helium in such exhaust would be minimized by having large quantities of helium stored atthe surface and introducing it as rapidly as possible, so that current-from the upcast shaft can be diverted as soon as possible into the conduit I5 and the discharge of such current to atmosphere simultaneously terminated.

I claim:

1. A closed circuit mine ventilating systemincluding mine workings, downcast and upcast shafts connected with said workings, means distant from the workings for discharging heat from gas in the circuit, and means for injecting into the circuit a respirable inert gas which comprises helium in a proportion of at least about 40% by volume.

2. A process of conditioning a mine atmosphere so as to minimize heating of the downcast ventilating current due to its autocompression, which process comprises the step of passing into the mine to provide the atmosphere therein a non-explosive respirable gas mixture containing the substantially normal atmospheric proportion of oxygen and in which the normal nitrogen content of atmospheric air is replaced by a diluent gaseous component selected from the group consisting of hydrogen, helium, and the nitrogenhelium mixtures containing at least 50% by volume of helium under conditions which preclude interaction between the components of said respirable miXture.

3. A process of conditioning a mine atmosphere so as to minimize heating of the downcast ventilating current due to its autocompression, which process comprises the step of passing into the mine to provide the atmosphere therein a non-explosive respirable gas mixture containing helium to at least about 40% of its volume.

4. A process of conditioning the atmosphere of a subterranean excavation having a depth of at least about 10,000 feet, which process comprises the step of passing into the excavation to provide the atmosphere therein at substantially atmospheric pressure at earth surface level a respirable ga mixture containing the substantially normal atmospheric proportion of oxygen and in which the normal nitrogen content of atmospheric air is replaced by a diluent gaseous component selected from the group consisting of hydrogen, helium, and the nitrogen-helium mixture containing at least 50% by volume of helium whilst preserving in said excavation conditions which preclude chemical interaction between the components of said respirable mixture.

5. A process of conditioning the atmosphere of a subterranean excavation having a depth of at least about 10,000 feet, which process comprises the step of passing into the excavation to provide the atmosphere therein at substantially atmospheric pressure at earth surface level a respirable gas mixture containing the substantially normal atmospheric proportion of oxygen and in which the normal nitrogen content of atmospheric air is replaced to at least of its volume by helium.

6. A process of conditioning the atmosphere of a subterranean excavation having a depth of at least 10,000 feet, which process comprises passing into the excavation to provide the atmosphere therein at substantially atmospheric pressure at earth surface level a respirable gas mixture containing the substantially atmospheric proportion of oxygen and in which the normal nitrogen content of atmospheric air is replaced to at least about 50% of its volume by helium, circulating said gas mixture through a closed circuit, cooling said mixture at at least one point of said closed circuit and removing from said mixture at at least one point of said closed circuit such carbon dioxide as may be in excess of the substantially normal atmospheric proportion of carbon dioxide.

JOHN PRICE REES. 

